Generally, a planographic printing plate is obtained by imagewise exposure, hardening the exposed portions and dissolving and removing the unexposed areas, and then conducting water-washing followed by a finisher gum treatment. In recent years, a method has been studied in which a planographic printing plate is digitally exposed based on image information employing laser rays, and developed with a developing solution to form images with high resolution and high sharpness. To list as one example, there is a system to manufacture a planographic printing plate by conducting direct scanning exposure onto a sensitized material, employing a light source modulated with image signals transmitted by communication line or outputted signals from an electronic plate making system or an image processing system.
However, a conventional planographic printing plate material employing a diazo resin has exhibited problems in that it was difficult to achieve spectral sensitization or speed enhancement in combination with the oscillating wavelength of laser light used for digital exposure.
Recently, a planographic printing materials used for planographic printing plates, having a photo-polymerizable light-sensitive layer containing a photopolymerization initiator, which can achieve enhanced speed by laser light have been noted for use in digital exposure using laser light. An enhancement of speed to shorten the recording time is desired for CTP (Computer To Plate) printing material in which digital data are recorded using a laser light source. Further, printing plates with improved press life have been desired in various printing fields including newspaper printing and commercial printing such as advertising media.
A photo-polymerizable light sensitive planographic printing plate employing an iron arene compound, triazine and coumarin has been known (for instance, please refer to Patent Document 1). A photo-polymerizable light sensitive planographic printing plate employing an iron arene compound, triazine and ketocoumarin has been also disclosed (for instance, please refer to Patent Document 2). Further, a photo-polymerizable light sensitive planographic printing plate employing a composition containing an iron arene compound and a tertiary amine monomer (for instance, please refer to Patent Document 3), and one employing a composition containing a metallocene, a trihalo compound, a tertiary amine, a dye and a binder (for instance, please refer to Patent Document 4) has been disclosed.
It is well known in the art that an iron arene compound is an effective cationic photopolymerization initiator and also a radical photopolymerization initiator (for instance, please refer to Patent Document 5). Further, it is known that image forming sensitivity is enhanced by using triazine or a sulfonium salt in combination with an iron arene compound as a sensitizer of that (for instance, please refer to Patent Documents 1, 2 and 6).
These combinations are preferable to exhibit a certain effect in terms of enhancing reactivity of a photo polymerization initiator, however these are not yet sufficient from the viewpoint of printing durability of a planographic printing plate.
On the other hand, as a technology to improve printing durability of a planographic printing plate, disclosed are planographic printing plates of the above mentioned Patent Document 5, and those employing a tertiary amine monomer, a PEG acrylate, a dye and a metallocene compound (for instance, please refer to Patent Document 7). These combinations are very effective in the viewpoint of enhancement of printing durability, however as a material correlating to the recent topical violet laser, these have a specific problem of safelight safety characteristics. Further, problems with cost or raw materials and safeness have emerged.
The inventors found as a result of diligent study, that the above problems could be solved by employing an iron arene compound as a photo polymerization initiator in combination with various photo acid generating agents, and thus, accomplished this invention.
Patent Document 1: Registered Japanese Patent No. 1986059 (Claims)
Patent Document 2: Registered Japanese Patent No. 3185585 (Claims)
Patent Document 3: U.S. Pat. No. 5,514,728 (Claims)
Patent Document 4: U.S. Pat. No. 5,049,479 (Claims)
Patent Document 5: International Publication No. 8802879 (Pamphlet)
Patent Document 6: JP-A 2001-100408 (Claims)
Patent Document 7: JP-A 10-306110 (Claims)